1. Field of the Invention
The present invention relates to centrifuges, and more particularly, to a centrifugal separator for solid liquid separation having a low-shear feeding system.
2. Description of the Prior Art
In a continuous flow centrifugal separator, a solid-liquid suspension in a feed stream is introduced into a rotating bowl. Various feeding systems have been employed to accelerate the velocity of the feed stream to the angular velocity of the bowl. Some prior art feeding systems were designed without consideration of the sensitivity of the solid particles in the feed to shear stresses. When a separator that incorporates such a feeding system is used to separate a solid from a solid-liquid suspension, the solid particles are typically subjected to high levels of shear stress. If the suspended particles are shear-sensitive, as in the case of precipitated proteins or living cells, the particles may be broken or otherwise damaged.
U.S. Pat. No. 5,674,174, issued to Carr (hereinafter xe2x80x9cthe ""174 patentxe2x80x9d), describes a feeding system that is intended to minimize shear stresses. The ""174 patent describes applying a feed stream to a rotating distributor cone by an applicator head in such a way that the velocity of the feed stream exiting the applicator head attempts to match the velocity of an adjacent rotating conical surface. However, in practice, as the feed stream contacts the rotating conical surface, it is subjected to a multi-dimensional velocity profile. There is a longitudinal component, e.g., a component parallel to the surface and normal to the direction of rotation, and one or more tangential components, i.e., components in the direction of rotation. In the ""174 patent, the applicator head imparts only a tangential velocity on the feed stream, and in many cases, shear stresses due to the longitudinal velocity component exceed those due to the tangential velocity component. Consequently, the applicator head of the ""174 patent does not produce sufficiently low shear stresses for use with mammalian cells. Also, in the system of the ""174 patent, the point on the rotating distributor cone at which the feed stream is applied is at a significant radial distance from the axis of rotation of the distributor cone, and as such, typical surface velocities are also significant. For example, if a feed stream is applied at a radius of 5 cm and the distributor cone is rotating at 10,000 rpm, the surface velocity that must be matched by the feed stream is approximately 5236 cm/sec. Imparting such a high velocity to the feed stream subjects the feed stream to a high level of shear stress in conduits leading to the applicator head. Additionally, a small mismatch in velocities between the feed stream from the applicator head and the spinning surface of the distributor cone, resulting either from the directional difference mentioned above, i.e., longitudinal versus tangential components, or from flow rate control tolerances, produces substantial shear stresses. Consequently, the system described in the ""174 patent appears to be best suited for suspended solids that are only moderately sensitive to shear, such as yeast cells or compact precipitates, but it is not suitable for more shear-sensitive materials, such as mammalian cells.
Another system that addresses the shear stress problem is disclosed in U.S. Pat. No. 5,823,937, issued to Carr (hereinafter xe2x80x9cthe ""937 patentxe2x80x9d). While the ""937 patent generally teaches placing a feed applicator off-center to an axis of rotation of a centrifuge bowl, it also describes a feed applicator that applies a feed stream concentric with the axis of rotation. The concentric approach, as compared to that of the ""174 patent, may reduce the radius from the axis of rotation at which the feed stream contacts the rotating surface and therefore potentially reduce shear stress. However, tests have revealed that concentric application of the feed stream, alone, does not guarantee that shear-sensitive materials are preserved.
Consequently, there is a need for a separator that is capable of processing the most shear-sensitive cells and precipitates. The present invention overcomes the problems associated with the conventional separator devices by providing a separator that is capable of processing ultra shear-sensitive cells and precipitates.
A centrifugal separator comprising (a) an accelerator rotatable at an angular velocity, xcfx89 about an axis, and having an inside surface with a point on the axis, and (b) a nozzle for introducing a feed stream at a volumetric flow rate, Q into the accelerator via an orifice. The orifice is substantially centered about the point, and the orifice has an inner diameter, d within the range of approximately
0 less than dxe2x89xa64xcex4,
where xcex4=1.414[(4Q/xcfx802xcfx89)1/3].